In air conduits of aircraft air-conditioning systems, in particular in the region of a discharge duct between the air conditioning pack and the mixing chamber, various forms of ice growth on the interior wall of the conduit occur under certain operating conditions. Without countermeasures this can result in complete closing of the duct cross section within a short time, which leads to undesirable performance characteristics of the air conditioning system. The critical operating conditions occur in particular during operation on the ground when the exterior air is warm and humid, when the air conditioning system needs to cool down a heated aircraft cabin from a high to a low temperature.
Nowadays, undesirable ice growth in ventilation ducts is usually prevented by periodically undertaken feed-in of hot air from the engine, so-called bleed air. Since there are uncertainties concerning ice formation, this also results in unnecessary heating of the discharge ducts, with such heating considerably reducing the efficiency of the air conditioning system and causing unnecessary energy loss.
In order to solve this problem, several concepts for detecting ice growth within conduits have so far been used on an experimental basis, however without achieving much success concerning reliable early detection or accurate ice thickness measurement. These concepts comprise, in particular, systems based on ultrasound sensor technology, in which systems it is, however, not possible to unequivocally determine the existing ice layers from the received signal echoes. It is impossible to differentiate between dense ice, loose ice snow and reflections from the opposite wall of the conduit. Temperature sensors that are used within the conduit cross section also do not provide an optimum solution because the temperature sensors themselves can ice up or can become damaged as a result of being bombarded by fast-flying ice particles. In addition, the flow within the conduit cross section is disturbed by the temperature sensors arranged therein so that the flow resistance to be overcome by the flowing air increases. Differential pressure measurement provides a further option for determining the thickness of ice layers in conduits, however in differential pressure measurement pressure measurement holes that lead outwards through the conduit wall close up as a result of ice formation, or pressure sensors installed within the conduit ice up. Accordingly, in principle all the measuring techniques based on the measuring of parameters within the conduit cross section are associated with the disadvantages of possible destruction of the sensors that are being used, or of their obstruction as a result of icing up or of bombardment by ice particles.